Stepping motor of the fluid pressure type

ABSTRACT

A step by step motor whose rotor is rotated by fluid pressure relatively to a stator. The invention provides a plurality of rows of rolling elements, either on the rotor or on the stator, arranged to co-operate with grooves formed in the other member, and actuating means for each row of rolling elements, the elements of successive rows being displaced with respect to the bottom of the grooves over an angular distance which varies by an increment corresponding to one step. Among other applications, control possibilities are provided for valves and machine-tool tables.

United States Patent i191 l Gachot et al.

im 3,844,178 Y [45] Oct. 29, 1974 [54] STEPPING MOTOR OF THE FLUID PRESSURE TYPE [76] Inventors: Jean Gachot, 179 Avenue de la Division Leclerc, Enghien-les-Bains; Simon Lekarski, 138 Boulevard de la Republique, Saint-Cloud, both of France [22] Filed: June 2, 1972 [21] Appl. No.: 259,033

[30] Foreign Application Priority Data June 3, 1971 France 71.20155 May 3, 1972 France 72.15665 [52] U.S. Cl 745/128, 92/58, 92/32 [5l] Int. Cl. Fl6h 27/00, FOlb 13/00 [58] Field of Search 92/58, 72, 32; 91/35, 36; 74/88, 128, 162, 144

[56] References Cited UNITED STATES PATENTS 2,134,072 10/1938 Christensen 92/98 D 2,715,925 8/1955 Morgan 74/144 2,874,544 2/1959 Switzer 60/97 SE 3,378,196 4/1968 Gudnestad 74/128 3,661,059 5/1972 Hunter et al 91/499 FOREIGN PATENTS OR APPLICATIONS 499,958 3/l95l Belgium 'I4/128 Primary Examiner-Edgar W. Geoghegan Attorney, Agent, or Firm-Young & Thompson [5 7 ABSTRACT A step by step motor whose rotor is rotated by fluid pressure relatively to a stator. The invention provides a plurality of rows of rolling elements, either on the rotor or on the stator, arranged to co-operate with grooves formed in the other member, and actuating means for each row of rolling elements, the elements of successive rows being displaced with respect to the bottom of the grooves over an angular distance which varies by an increment corresponding to one step. Among other applications, control possibilities are provided for valves and machine-tool tables.

6 Claims, 11 Drawing Figures STEPPING MOTOR OF THE FLUID PRESSURE TYPE Y This invention relates to a stepping motor which operates by fluid pressure.

The stepping motors which are in widest use are often subdivided into three main groups, namely electric motors, hydraulic motors and pneumatic motors.

The motors of the first group have a very fast action and produce low values of torque. The motors of the second type are usually associated with electric motors and perform the function of amplifiers with respect to these latter since they are not capable of operating in an independent manner. So far as the pneumatic motors are concerned, they comprise in most cases a piston and ratchetwheel mechanism which is unreliable and makes it possible to obtain only a low torque.

Moreover, these different types of motors do not provide any information relating to the ends of stepping intervals; in consequence, any errors in step displacements which may arise, especially steps which have not been performed, cannot be located.

The aim of the invention is to overcome the different disadvantages mentioned in the foregoing and to provide a stepping motor which has an extremely high degree of reliability.

In accordance with the invention, the stepping motor which operates by fluid pressure and comprises two members capable of moving with respect to each other is characterized in that one of the members is fitted with a plurality of rows of rolling4 elements adapted to cooperate with depressions formed in the other member, and means for actuating each row of elements, the elements of successive rows being displaced with respect to the bottom of the depressions at an angular distance which varies by a value corresponding to one step. l

The sequential actuation of the rows of rolling elements makes it possible to impart to the moving member displacements which are equal in value to one step. The cooperation of rolling elements with complementary depressions ensures reliable operation with the maximum degree of accuracy.,

The depressions preferably consist of identical successive grooves arranged in parallel relation. The actuating means are pistons controlled by the fluid pressure. In the case of a rotating motor, the two members being movable in relative rotation, the grooves and the rows of rolling elements are oriented parallel to the axis of rotation, the step being equal to the ratio of the angle made between two consecutive grooves to the number of rows.

The actuation of a single row of rolling elements or of two opposite rows serves to initiate a displacement of the rotary member by one step.

ln the case of a linear motor, the two members being movable in relative translational motion, the rows of rolling elementsare oriented in the direction of translational motion and the grooves are oriented at right angles to this direction, the step being equal to the ratio of the distance between two consecutive grooves to the number of rows. Preferably, `the rows of `rolling elements are arrangedin two symmetrically disposed series in which the rolling elements are inclined at a predetermined angle with respect to the grooves and transversely to the direction of translational motion.

In order that the sequential control should be performed without any omission or error in step, the actuating means are combined with means for generating end-of-travel signals which are connected to a control logic circuit.

In an advantageous embodiment of the invention, provision is made for means whereby the two members are brought both accurately and automatically into a predetermined relative position by means of data which are coded in accordance with a predetermined law.

To this end, the motor comprises a system for detecting the instantaneous relative position of two moving members adapted to deliver position signals which are coded in accordance with a decimal law, a positioningorder generator adapted to deliver primary reference signals which are coded in accordance with a predetermined law and represent the desired relative position of the two moving members, a transcoding system adapted to produce from said primary reference signals secondary reference signals which are coded. in accordance with a decimal law, and a comparison system adapted to receive on the one hand the position signals and on the other hand the secondary reference signals and to deliver actuating signals as a function of the relative value of the two series of signals aforesaid to the logic circuit which controls the motor.

Further particular features of the invention will become apparent from the description which is given hereinafter:

-In the accompanying drawings, which are given by way of example and not in any sense by way of limitation:

FIG. l is a transverse sectional view taken along line l-I of FIG. 2 and showing a stepping motor of the rotary type;

FIG. 2 is a sectional view of said motor, taken along lineII-Il of FIG. l;

FIG. 3 is a detail view toa larger scale and illustrating the relative angular displacement of the rows of balls with respect to the grooves of the r otary member;

FIG. 4 is a diagram representing the motor together with its control logic circuit;

FIG. 5 is a partial longitudinal sectional view taken along line V-V of FIG. 6 and showing a stepping motor of the linear type;

FIG. 6 is a partial sectional view with portions broken away and taken along line VI-VI-of FIG. 5;

FIG. 7 is a sectional view taken along line VlI--VII of FIG. 6;

FIG. 8 is a view to a larger scale showing a detail of FIG. 5;

FIG. 9 is a sectional view which is similar to FIG. 7 and shows a stepping motor of the linear type having two stages;

FIG. l0 is a diagrammatic view showing the arrangement of the position detectors;

FIG. ll is a diagram of the transcoding and compari son system.

FIGS. l and 2 illustrate a stepping motor of the rotary type comprising a stationary member l which is rigidly shaped cross-section and each containing a piston 6 provided with an actuating rod 7. Said chambers and pistons 6, which are eight in number in the example under consideration, are disposed radially with respect to the axis X-X and in uniformly spaced relation. The piston rods are in contact with rows R1 to Rg of balls 8 which are housed respectively in slots 9 formed in a sleeve 10 which is secured to the body 4.

The rotary member 3 is made up of two rotating endbosses 1l which are mounted in the frame 2 and of a sleeve l2 of hard metal which is fixed on said endbosses. Identicalv V-section grooves 13 which are formed over the entire periphery of the sleeve 12 and oriented parallel to the axis of rotation X-X permit the engagement of the rows of balls R1 to Rs to a variable depth, as will be explained hereinafter.

Each piston 6 is rigidly fixed to a diaphragm`14 which is mounted in the corresponding chamber S and clamped between the body 4 and a cover 15 in which is formed a passageway 16 having its opening in the volurne defined by the diaphragm and the cover. The piston 6 is capable of displacement within the chamber 5 over a short distance determined by the depth of penetration of the rolling element within the groove 13, thus defining the maximum travel. The covers are held in position by means of intermediate longitudinal components 17 which are attached to the body 4.

The arrangement is such that the axes of the balls of the successive rows are displaced with respect to the bottom of the respective grooves 13 at an angular distance which varies from one row to another by a value corresponding to the single-step forward displacement of the motor. This single-step displacement is equal to the ratio of the angle 6 between the roots of two consecutive grooves (angular pitch of the grooves) to the number N of rows of balls (as shown in FIG. 3).

ln the example considered, one of the rows of balls, namely the row R, taken as a reference, is in the position of maximum engagementl within the corresponding groove whilst the row R5 which is located directly opposite is displaced vby 6/2 inasmuch as N 8. The balls of the last-mentioned row are therefore in a position of withdrawal on a summit or tooth between two grooves 13. ln order to initiate a one-step rotary displacement of the moving member 3, compressed air is delivered through the passageways 16 into the chambers 5 of the pistons 6 corresponding to the rows of balls R2 and R6,v

for example. The balls of the rows are angularly displaced through an angle 0/8 so that the second row of balls R2 should then take up the position of maximum engagement which was previously occupied by the row having the reference R1.

FlG. 4 illustrates the sequential control of the actuating pistons 6 by means of a storage-type logic circuit.

The chambers of the pistons are connected in opposite pairs to two-state storage devices 2l-24 of conventional type having two inputs, namely a control and storage input and a cancelling or erasing input, and one output. The pneumatic control signals are delivered from one of two lines 30a, 30b, depending on the direction of rotation which is selected for the moving member 3. These lines are combined in pairs with four lines 31-34 of end-of-travel signals corresponding respectively to the pairs of pistons and therefore to the rows Rl R5, R2 R6, and so forth. Each pair formed by the line 30a or 30h and a line 31, 32, 33 or 34 is thus connected to an AND-gate 4la-44a, 41b-44b. The output of each AND-gate is connected on the one hand to an OR-gate 51-*54 corresponding tothe pair of pistons to be actuated and, on the other hand, to an OR-gate 61-64 corresponding to the preceding pair of pistons (depending on the direction of rotation) which has been actuated; the association of the AND-gates and OR-gates 51-54 is effected in the following order: AND-gate 44a and 42b OR-gate 51, AND-gate 4la 43b OR-gate 52, and so forth.

A signal derived from one of the pairs of lines 30a-34 and 30h-32, 30a-3l and 30b-33 and so forth is accordingly applied to the OR-gates 51-54 respectively whilst a signal dervied from a pair of lines 30a-3l or 30b-31, 30a-32 or 30b-32 and so forth is applied to the OR- gates 61-64 respectively.

Each OR-gate 51 to 54 (for example, the gate 5l) corresponding to one pair of rows (Rl and R5, for example) is connected to the control and storage input of the storage device 21-24 (device 21, for example). The OR-gate 6l to 64 (namely the gate 64 or 62, for example) corresponding to ythe preceding pair of rows (R2 and R4 or R2 and R2, for example) is connected to the erasing input of the storage device 21-24 (either 24 or 22, for example).

The end-of-travel signals in the lines 31-34 are delivered under the action of the pistons 6 by means of conventional means for generating end-of-travel signals (which are described hereinafter) with which the pistons are combined.

When an end-of-travel signal and a rotational direction signal appear at an AND-gate, this gate delivers a signal to the OR-gate 51-54 and to the corresponding OR-gate 61-64. The signal is transmitted from the OR- gate 51-54 to the respective storage device 21-24, then to the chambers of the pistons corresponding to the pairs of rows R1 R5, R2 R6, and so forth. The signal derived from the OR-gate 61-64 has the effect of erasing the information contained in the storage device corresponding to the pair of rows which has previously been actuated.

The control of the motor is ensured without any risk of error and with a remarkable degree of regularity since each end of travel constitutes an item of information which is necessary in order to permit of sequential actuation.

FIGS. 5 to 8 illustrate a stepping motor of the linear type comprising a stationary member 71 constituted by a plate and a member 72 which is capable of translational motion and essentially composed of a table 73 such as a machine-tool table, for example, which is fitted with two lateral components 74 guided by means of slideways 75 formed by the plate 7l.

The plate 7l is provided on its top face with a series of identical grooves 76 having a V-shaped cross-section and oriented at right angles to the direction of translational motion. l

There is mounted between the plate 71 and the table 73 a support-block 77 which is rigidly fixed to the table 73 and to the lateral components 74, there being formed in said block individual cyclindrical chambers 78 for the pistons 79 which are disposed at right angles to the plate 7l. Each piston 79 is in contact with a ball 80 which is housed in a bore formed in a thin plate 8l which is rigidly fixed to the support-block 77 and located above the grooved plate 71. The balls 80 can thus engage to a greater or lesser depth within the grooves The table 73 is provided in its central portion with a recess 82 in which is mounted an added component 83, and with a transverse rib 84 which divides the table into two halves. The chambers 78, the pistons 79 and the corresponding balls 80 are disposed symmetrically in columns on each side of said rib in such manner as to be inclined in the transverse direction at a predetermined angle with respect to the grooves 76. In this example, there are thus formed two series of seven rows of pistons and of balls as designated by the references L, to L1 and L', to L,. The pistons 79 are secured to a diaphragm 85 which is clamped between the table 73 and the supportblock 77 and each row L, L7 or L', L', is supplied by a passageway 86 or 86' which is formed in the table 73 and interrupted by the transverse rib 84. Passageways'87 are formed in the added component 83 so as to establish a communication between the passageway 86 of the row L, and the passageway 86 of the row LQ, betweenthe passageway 86 of the row L, and the passageway 86' of the row L', and so on up to the passageways 86 and 86' of the rows L, and L,.

The arrangement is such that, in each series, the balls 80 of the successive rows L, L7 or L', L', are displaced with respect to the bottom of the respective grooves 76 at a distance which varies by a value corresponding to one step. This value is equal to the ratio of the distance e between two consecutive grooves 76 to the number of rows (seven in the case under consideration); this determines the angle of inclination of the columns of balls 80 with respect to the grooves 76 in the transverse direction.

Each passageway-86 which supplies the rows of balls L, to L7 has an extension in the form of a stepped bore 88 constituting a passage which serves to supply fluid under pressure to the passageway 86 aforesaid as well as to the corresponding passageway 86', and consequently to actuate the two corresponding rows of balls.

There are also formed in the support-block 77 ducts 91 which are equal in number to the rows and open into respective bores 92 which are oriented at right angles to the grooved plate 71, and said bores being normally closed-off by a valve V93 mounted on a blade-spring 94 which is secured to the support-block 77.

The free end of the blade-spring is in contact with the underface of the first piston 79 of the row, in such manner that the displacement of said piston under the action of the fluid pressure causes the valve 93 to open and connects the bore 92 and the duct 91 to the exhaust. The ducts 88 and 91 are connected to a fluidcontrol logic circuit (not shown in the drawings) which produces action in response'to the absence of pressure v within a duct 9l as a result of connection of this latter FIG. 9 illustrates a linear stepping motor having two stages, in which the grooved plate 101 of the top assembly performs the function of table for the bottom assembly. The structure of both assemblies is in any case identical.

The bottom stage is intended to carry out a one-step displacement over a relatively long distance such as l mm, for example, whereas the top stage must permit a one-step displacement over a very short distance such as 0.05 mm, for example.

The relative spacing of the grooves of the plate 101 is equal to the distance between two columns of balls in the bottom stage. In the example considered, it is therefore possible to initiate a displacement of the table 73 over a predetermined number of millimeters by virtue of the top stage, and over` multiple of 0.05 mm by virtue of the top stage. t

The two stages are supplied separately with fluid .under pressure by means of a control logic circuit. If

necessary, the two stages can operate simultaneously, especially in a numerical positioning.

lt is apparent that the number of stages can increase according to the degree of precision which is required in the displacement of the moving member, this being essentially dependent on the type of industrial application which is contemplated. n r g It is also apparent that the structure and operation of the linear motor embodiments of the present invention are substantially the same as those of the rotary motor embodiments, except that the linear motor is in effect a flat form of the rotary motor. In other words, the linear motor is in effect one section of a rotary motor of infinite radius. Accordingly, it is not necessary to repeat, in connection with the linear embodiments, what has already been said in connection with the rotary embodiments. V

The use of the stepping motor of the rotary type is particularly advantageous for the control of valves and other devices which can entail the need for limited angular displacements of rotating components.

By way of alternative, the depressions can consist of conical recesses instead of grooves.

There will now be described with reference to FIGS. l0 and l1 a system which permits accurate positioning of the two members of the motor with respct to each other by means of data which are coded in accordance with a predetermined law.

In this system, the selected number N of pistons and rows of balls is equal to ten. Under these conditions, the single-step forward displacement is equal to onetenth of the angular or linear distance (depending on whether a rotary or linear motor is employed) between two consecutive grooves 13, said distance being such as to correspond to the pitch of the depressions. There is shown diagrammatically in FIG. l0 a device for detecting the instantaneous position of the moving member 1l. The grooves 1'3 are shown in the developed position with the first piston P., and the last pistonl between the axis of the origin groove 13r and the reference position 140-of the grooves.

In the position shown in FIG. l0, the axis 140 of the row of balls 8a is displaced by one step, that is tofsay by d/lO wherein d designates the grooves pitch, with respect to the axis of the corresponding groove 13a whilst the axis of the row of balls 8b corresponding to the piston P9 coincides with the axis of the groove 13b. Assuming by way of example that the groove 13a is the eighth as counted in the direction of the arrow f from the origin groove 13 it is apparent that the position shown is defined by 79 forward-displacement steps.

The system of detection comprises a first series of ten fluid-pressure detectors A0, A1, A9 which are actuated by the pistons P when they-are located in a reference position. This reference position preferably coincides with the end-of-travel position of the piston.

Thus, in the example illustrated in FIG. 10, it is the detector A9 which is actuated and emits a position signal. Forthe sake of convenience of terminology, these first detectors will be designated hereinafter as unit detectors and the signals delivered by these latter will be referred-to as 'unit position signals. The intervals between these unit signals correspond to one singlestep displacement of the motor.

The position-detection system further comprises .a second series of ten detectors B0, B', B9 which will be designated hereinafter as tens detectors. The detectors just mentioned are actuated either by the teeth themselves which are formed between the grooves 13 or by a cam or any equivalent member so as to emit a signal referred-to as a tens" position signal when the groove 13 which corresponds thereto is in the selected reference position. It is clearly an advantage to choose as reference position for the grooves a position corresponding to the axis 140 of a piston P0. The detector B0 corresponds to the groove 13r which is chosen as origin. The intervals between the tens signals correspond to l single-step displacements of the motor, that is to say to one pitch of the grooves 13.

lf the number of depressions or grooves 13 is greater than ten, the system of detection comprises a third series of ten detectorsCU, C, C9 or so-called "hundreds" detectors. Each detector C is actuated by a cam or like component having a boss, the position of which corresponds to that of a set of l0 consecutive grooves 13. The detectors Ct emit a signal when the set of l0 grooves corresponding thereto reaches the reference positionof the grooves. ln consequence, the interval between said hundreds signals corresponds to one hundred single-step displacements of the motor or to lO depression-pitch intervals.

The detection system can also comprise decimal series of detectors having a higher order if the number of grooves 13 so requires, the interval between the signals o f one series being such asto correspond to a forward displacement ten times greater in value than that which corresponds to the interval between the signalsv of the series immediately below. l f i The outputs of theA unit detectors A are connected to lines U0 to Ug'which terminate in a first comparator 111 (FIG. l1`) of a type which is known per se.

Similarly, the outputs of the tens detectors B are connected to lines D0 to D9 which terminate in a second comparator 112 which is similar to the first comparator.

For convenience of terminology, the comparators 111 and 112 will be designated respectively in the following discussion as unit comparator and tens comparator.

Although the number of comparators is limited to two forthe sake of clarity of the drawings and of the description, it is wholly apparent that these comparators are equal in number tothe decimal series of position detectors.

An order generator which is not illustrated and known per se is designed to emit primary reference signals representing the desired position for the moving member expressed in forward-displacement steps as stated earlier.

This number of forward-displacement steps which defines the desired position is represented in accordance with a predetermined code by the primary reference signals which appear on lines such as 113 to 120, the number of which depends on the code adopted. Said lines terminate in a transcoder of a type which is known per se and which, for the sake of enhanced clarity ofthe drawings, is illustrated in FIG. 11 in two separate sections 121 and 122. In a manner which is already known, the transcoder decodes theinformation represented by the primary reference signals and recodes said information in accordance with a decimal law so as to emit secondary reference signals. These secondary signals comprise a unit signal which appears on one of the lines 13 to E9 which are connected to the unit, comparator 111 and a tens signal which appears on one of the lines F0 to F9 which are connected to the tens comparator 112.

It is readily apparent that, as a general rule, the transcoder comprises a number of sections corresponding to the number of comparators.

Each comparator 111 and 112 has two outputs and is arranged in a manner known per se so as to produce an actuating signal which appears either on a line 123 or 124 respectively if the position signal which appears on one of the lines D or U is of lower strength than the reference signal delivered by one of the lines Eor F, or on a line 125 or 126 in the contrary case. If the position signal is equal to the reference signal, the comparators do not emit any actuating signal.

The lines 123 and 124 are each connected to one input of an OR-gate 127 whilst the lines 125 and 126 are connected to the inputs of a second OR-gate 128.

The outputs of these OR-gates are connected to the` lines 30a and 30b of the motor-control logic circuit which was described in detail earlier.

This logic circuit is illustrated in FIG. 4 for a number of pistons 6 equal to eight. Within the framework of the present embodiment, it is wholly evident that, if the number of pistons is equal toten, the circuit must accordingly be extended in a manner which is well known to any one versed in the art in order to adapt that circuit to thisnumber of pistons.

f It will be noted in addition that the unit detectors A are connected not only to the lines U of the units comparator lll but also to the lines 31 to 34 of FIG.` 4 in order to deliver to the motor-control logic circuitv the end-of-travel signals which were mentioned in the foregoing.

The tens comparator 112 of FIG. 11 is supplied directly from two lines 129 and 130. The supply of the units comparator 111 is controlled by the actuating signals emitted by the tens comparator 112 through an inhibition circuit. This circuit comprises an OR-gate 131 and an inverter or NOT-gate 132 which constitutes in combination a NOR-gate. The OR-gate 131 having two inputs connected respectively to the output lines 123 and 125 of the tens comparator and the output of the NOT-gate 132 is connected to the inputs for the supply of the units comparator lll. lt is understood that, under these conditions, the units comparator can emit a signal only if no signal is present either on the line 123 or on the line 125.

It is readily apparent that, as a general rule, each comparator is controlled by the comparator having the order immediately above through an inhibition circuit which is similar to that described in the foregoing.

The device which has just been described operates as follows:

ln order to simplify the explanation, it will be assumed hereinafter that the position-detecting system comprises only unit detectors A and tens detectors B whilst the comparison system similarly comprises only one units comparator 111 and one tens comparator 112, as shown in FIG. 1l. The extension to the general case is evident to any one versed in the art.

lf it is assumed that the desired position of the moving member of the motor corresponds to a distance of twenty-eight single-step displacements between the groove 13r which is chosen as origin and the axis 140 which is chosen as reference position ofthe grooves, the order generator (not shown) emits on the lines 113 to 120 a reference signal which represents this number in a selected code. After decoding and recoding of said signal by the transcoder 121, 122, there appears on the line E8 a signal which represents-the number eight and on the line Fza singal corresponding to the second multiple of ten, that is to say to the number twenty. These two signals are indicated schematically by points on the corresponding lines.

When the motor is in the position shown in FlG. l0, the tens detector B1 corresponding to the groove preceding the groove 13a is actuated and delivers a signal on the line D7. ln addition, the unit detector A9 emits a signal on the line U9 since it is the piston P9 which is located at the end of travel. These two signals represent the number 79 which defines in forwarddisplacement steps the actual instantaneous position of the moving member as has been stated earlier. These signals are presented by the letters X on the corresponding lines.

The tens" comparator 112 records the fact that the position signal which it receives on its line D7 is of higher strength than the reference signal which appears on the line F2. ln consequence, the comparator produces an actuating signal on its output line 125. This signal is transmitted to the OR-gate 128 and thence to the line 30b of the control logic circuit which causes the motor to advance in the appropriate direction.

At the Sametime, said actuating signal is brought to the inhibition circuit 131, 132 which cuts-off the supply of the units comparator 111 and this latter can therefore not deliver any signal.

As the motor advances, so its position is detected by tens signals which appear successively on the lines D6, D and so forth. When the "tens" signal appears on the line D2, the tens" comparator 112 no longer emits an actuating signal. The position of the motor is defined at this moment by the number 29.

Since the inhibition circuit no longer receives any singal, it permits the supply ofthe units comparator 111. This comparator records the fact that the position signal which it receives on the line U9 is of higher strength than the reference signal which is displayed on the line E8. ln consequence, said comparator produces an actuating signal on its output line 126 which, through the OR-gate 128, causes the motor to advance in the same direction as before up to the desired position which is defined by the number 28. When this desired position is reached, the units comparator 111 no longer emits its actuating signal and the motor stops.

lt is understood that the device which has just been described permits accurate control of the motor by very simple means. In particular, the choice of the number of pistons and the distribution of the position detectors make it possible to obtain instantaneousposition signals which are coded directly in accordance with a decimal base and avoid the need to make use of a special transcoder.

What we claim is:

1. A stepping motor comprising two members capable of moving with respect with each other, one of said members having cage means retaining a plurality of successive rows of rolling elements that engage in successive depressions in the other member, and fluid pressure actuating means for each said row to selectively press only one row of said successive rolling elements to the bottom of one of said depressions at one time, one row (R1) being at the bottom of one said depression (13) at one time, and each of the other successive rows (R2-R8) of rolling elements (8) being offset with respect to each of the corresponding successive depressions (13) by a distance which is equal to the pitch of the depressions (13) divided by the total number of said rows (R1-R8) of rolling elements (8), whereby only one said actuating means moves said members relative to each other through a said distance at one time.

2. A motor according to claim 1 wherein, the two members being movable in relative translational motion, the rows of rolling elements are oriented in the direction of translational motion and the depressions are oriented at right angles to this direction, the step being equal to the ratio of the distance between two consecutive depressions to the number of rows.

3. A motor according to claim 2, wherein the rolling elements form columns disposed transversely to the direction of translational motion. g 1 l 4. A motor according to claim 2, wherein the rows of rolling elements are distributed in two symmetrically arranged series in which the rolling elements considered transversely to the direction of translational motion are inclined at a predetermined angle with respect to the depressions.

5. A motor according to claim 4, wherein the rows of the two series are supplied with fluid under pressure through passageways which are put symmetrically into communication with each other in pairs.

6. A motor according to claim 2, wherein said motor comprises at least one second member fitted with rolling elementsin cooperating relation with depressions formed in the first member so as to constitute a motor having at least two stages. 

1. A stepping motor comprising two members capable of moving with respect with each other, one of said members having cage means retaining a plurality of successive rows of rolling elements that engage in successive depressions in the other member, and fluid pressure actuating means for each said row to selectively press only one row of said successive rolling elements to the bottom of one of said depressions at one time, one row (R1) being at the bottom of one said depression (13) at one time, and each of the other successive rows (R2-R8) of rolling elements (8) being offset with respect to each of the corresponding successive depressions (13) by a distance which is equal to the pitch of the depressions (13) divided by the total number of said rows (R1-R8) of rolling elements (8), whereby only one said actuating means moves said members relative to each other through a said distance at one time.
 2. A motor according to claim 1 wherein, the two members being movable in relative translational motion, the rows of rolling elements are oriented in the direction of translational motion and the depressions are oriented at right angles to this direction, the step being equal to the ratio of the distance between two consecutive depressions to the number of rows.
 3. A motor according to claim 2, wherein the rolling elements form columns disposed transversely to the direction of translational motion.
 4. A motor according to claim 2, wherein the rows of rolling elements are distributed in two symmetrically arranged series in which the rolling elements considered transversely to the direction of translational motion are inclined at a predetermined angle with respect to the depressions.
 5. A motor according to claim 4, wherein the rows of the two series are supplied with fluid under pressure through passageways which are put symmetrically into communication with each other in pairs.
 6. A motor according to claim 2, wherein said motor coMprises at least one second member fitted with rolling elements in cooperating relation with depressions formed in the first member so as to constitute a motor having at least two stages. 